This invention relates generally to semiconductor devices. More specifically, the invention relates to large area, thin film semiconductor devices such as photovoltaic devices; and most specifically, the invention relates to a method for scribing portions of a large area semiconductor device.
Advances in semiconductor technology have made possible the preparation of large area, high quality, thin film semiconductor materials by high volume deposition techniques. This has made possible the manufacture of very large area semiconductor devices such as photovoltaic devices, photo sensor arrays, memory arrays, displays and the like. Such devices typically comprise a number of layers of semiconductor material, insulator material and electrode material in various stacked relationships; and, in the course of fabricating semiconductor devices, it is necessary to scribe various of these layers. For example, very large area photovoltaic devices are often scribed to configure the materials into interconnected series, parallel, or mixed series/parallel arrays of subcells. Likewise, scribing is necessary in the fabrication of other device arrays.
Photovoltaic devices, and many other semiconductor devices, comprise one or more layers of semiconductor material interposed between a bottom electrode material and a top electrode material, and it is often necessary or desirable to scribe the top electrode material into a plurality of separate regions. Owing to the relatively high lateral electrical resistivity of thin film semiconductor materials, such scribed regions are effectively electrically isolated. Accordingly, a number of techniques have been implemented in the art for scribing such materials. Scribing can be carried out by chemical processes such as etching or by mechanical processes such as water jet abrasion.
Laser techniques are particularly preferred for scribing large area semiconductor devices since such techniques can form relatively small features, and since the position of the laser beam can be controlled with a high degree of precision. Laser techniques are known in the art; for example, such techniques are shown in U.S. Pat. Nos. 5,268,037 and 5,457,057. While a laser beam may be accurately positioned, control of delivered power can present problems. The output power of the laser can fluctuate, unless expensive power supplies and regulators are employed. Also, minor variations in the thicknesses of the relatively thin semiconductor and electrode layers may occur thereby causing irregularities in scribing even if the laser output is highly stable. Excess laser power can damage subjacent and adjacent portions of semiconductor material thereby short circuiting a semiconductor device or rendering it otherwise inoperable. If the power delivered is too low, scribing may not be complete and electrical isolation compromised also resulting in an inoperative device.
There is thus a need for a method for scribing large area semiconductor devices, and particularly top electrodes of large area semiconductor devices, which method is accurate and precise with regard to geometric positioning, and which is capable of producing consistent scribing results. This method should also be compatible with conventional processing techniques and equipment, and should be fast and simple to implement. As will be described hereinbelow, the present invention provides a highly controllable, laser based, accurate, scribing method which may be readily adapted to techniques for the fabrication of various large area semiconductor devices.
There is disclosed herein a method for scribing the top electrode of a semiconductor device which device includes a top electrode, a bottom electrode and a body of semiconductor material disposed therebetween. In a first step of the method, a scribed area is defined on the top electrode. The scribed area is separated from the remainder of the top electrode by a high resistance portion of the top electrode. This high resistance portion has a lateral, or sheet, resistance which is greater than the electrical resistance of the top electrode in either the scribed area or the remainder of the device. In particular embodiments of the invention, the high resistance portion of the top electrode is formed by directing a laser beam onto the top electrode so as to remove at least some of the top electrode material thereby forming the high resistance portion.
In a second step of the method, the electrical resistivity of a portion of the semiconductor body is decreased, and this portion of decreased resistivity provides a lower resistivity path which extends between the bottom electrode and the scribed area of the top electrode. In particular embodiments of the invention, the decreased resistivity of the semiconductor body is accomplished by heating the semiconductor body, for example by laser heating.
Subsequently, the semiconductor device is disposed in an electroconversion bath and an electrical current is flowed through the device from the bottom electrode, through the portion of semiconductor-decreased resistivity, and through the scribed area of the top electrode. This current causes the electroconversion bath to oxidize the top electrode material in the scribed area thereby making it electrically resistive.